The present invention relates generally to integrated circuit memory devices and, more particularly, to an apparatus and method for implementing precise sensing of phase-change random access memory (PCRAM) devices.
Dynamic Random Access Memory (DRAM) integrated circuit arrays have been in existence for several years, with their dramatic increase in storage capacity having been achieved through advances in semiconductor fabrication technology and circuit design technology. The considerable advances in these two technologies have also resulted in higher and higher levels of integration that permit dramatic reductions in memory array size and cost, as well as increased process yield.
A DRAM memory cell typically includes, as basic components, an access transistor (switch) and a capacitor for storing a binary data bit in the form of a charge. Typically, a first voltage is stored on the capacitor to represent a logic HIGH or binary “1” value (e.g., VDD), while a second voltage on the storage capacitor represents a logic LOW or binary “0” value (e.g., ground). A basic drawback of a DRAM device is that the charge on the capacitor eventually leaks away and therefore provisions must be made to “refresh” the capacitor charge, otherwise the data bit stored by the memory cell is lost.
The memory cell of a conventional Static Random Access Memory (SRAM), on the other hand, includes, as basic components, an access transistor or transistors and a memory element in the form of two or more integrated circuit devices interconnected to function as a bistable latch. An example of such a bistable latch is a pair of cross-coupled inverters. Bistable latches do not need to be “refreshed,” as in the case of DRAM memory cells, and will reliably store a data bit indefinitely so long as they continue to receive supply voltage. However, such a memory cell requires a larger number of transistors and therefore a larger amount of silicon real estate than a simple DRAM cell, and draws more power than a DRAM cell. Like a DRAM array, an SRAM array is also a form of volatile memory in that the data is lost once power is removed.
Accordingly, efforts continue to identify other types of memory elements that are capable of storing data states, that do not require extensive refreshing, and that are non-volatile in nature. Recent studies have focused on resistive materials that can be programmed to exhibit either high or low stable ohmic states. A programmable resistance element of such material could be programmed (set) to a high resistive state to store, for example, a binary “1” data bit or programmed to a low resistive state to store a binary “0” data bit. The stored data bit could then be retrieved by detecting the magnitude of a readout voltage generated when a current is switched through the resistive memory element using an access device.
Phase Change Random Access Memory (“PCRAM” also referred to as “PRAM”) is an emerging non-volatile memory technology which stores data using phase change materials (such as Ge—Sb—Te (GST) alloys) having a programmable electrical resistance that changes with temperature. Other compositions such as GeSb4, (including substitution/addition of other elements) are also possible for the phase change materials. Individual phase change elements (PCE) are thus used as the storage cells of a memory device. The state of an individual PCE is programmed through a heating and cooling process which is electrically controlled by passing a current through the PCE (or a discrete heating element in proximity to the PCE) and the resulting ohmic heating that occurs. Depending upon the specific applied temperature and duration of heating applied to the PCE element, the structure is either “set” to a lower resistance crystalline state or “reset” to an amorphous, higher resistance state.
Data is read from a given PCRAM cell by selecting a bit line and a word line for that cell, passing a current through that PCRAM cell, and thereafter distinguishing a “1” from “0” based upon the voltage generated from the variable resistance of the phase change material of the PCRAM cell. Because the resistance of a phase change element may be as low about 1 kΩ or as high as about 10 MΩ, the current must be controlled with precision across a very large range. However, existing current sense schemes either lack the range or precision required for robust phase change element resistance measurement throughout the full resistance range. Accordingly, it would be desirable to be able to implement an improved sensing and resistance measuring technique for phase-change random access memory (PCRAM) devices, as well as for other types of memory devices.